A known method of making a flat ceramic tile substrate is "roll compaction pressing." In this method ceramic powder is placed in a hopper and is continuously fed between two rollers rotating in opposite directions to form a tape. Only one powder material is loaded into the roll compaction press at one time. After pressing, the tape is cleaned and cut into tiles of various sizes for a particular process or component. However, it was not heretofore known if such a process has been used in making an automotive exhaust sensor.
Two flat zirconia tile substrates may be utilized to make an exhaust sensor. One tile will have platinum electrodes printed on opposite faces and will be the sensor tile. The other tile will have a dielectric film printed on one side and a heater pattern printed over it. This tile will be the heater tile. The dielectric is printed on the one side of the heater tile to prevent electrical shorting from the heater to the sensor. On the inside of the heater tile an air reference slot is made. The two tiles will be laminated together in a process using heat and pressure to form a tile that has a multiple of sensor elements on it.
However, the use of zirconia for the sensor substrates is costly. More economical materials such as alumina have been utilized in conjunction with zirconia. U.S. Pat. No. 4,574,042 discloses a gas analyzing apparatus including an alumina substrate and a solid zirconia electrolyte disk loosely mounted in a circular groove provided near one end of the alumina substrate and held in place by inserting a platinum O-ring between the alumina substrate and the zirconia disk. Such devices are difficult to make and suffer from thermal expansion stresses due to the difference in thermal coefficient of expansion of alumina and zirconia. There are also more suitable substrate materials that have superior thermal and mechanical properties such as alumina.
Moreover, the use of a thin dielectric coating or tape has been proven to be insufficient to prevent electrical shorting. Any porosity or microcracking in the coating ultimately leads to shorting of the device, particularly when using a braze to obtain interconnection to the substrate. Zirconia being conductive at elevated temperatures, can lead to shorting, again particularly when using a braze to obtain interconnection to the substrate.
Substrates made from zirconia are generally weak and have poor thermal shock resistance. They can also exhibit mechanical degradation due to phase transformations if the chemistry, grain size, and sintering conditions are not explicitly controlled.
Zirconia is also a difficult material to process into tape. Special care and preparation of the material is required in order to form the material into a usable tape. Much less care and attention is required of alternate materials such as alumina. This also has an impact on the cost of both equipment and manpower to produce tape.
The present invention overcomes the above disadvantages of the prior art.